A computational study of calcium cation locations and diffusion in chabazite

The characteristics of calcium ions in high-silica chabazite are explored by treating the aluminium and calcium ions as a defect in an otherwise purely siliceous bulk with the defect modelled using the Mott-Littleton method. Three generic sites are found for calcium, which correspond to the centre of the double six-ring unit, on the triad axis of the double six-ring unit but displaced into the cavity and in the eight-ring window. Distortion of the framework is also examined. The main energetic barriers to cation migration are found. As expected these are large in relation to kT, even at elevated temperatures, and significant diffusion of the cations can be ruled out for anhydrous chabazite. (C) 1999 Elsevier Science B.V. All rights reserved.